Wave-dependent drag parametrizations and their impact on drag and water levels

In this report two aspects of wave-dependent drag parametrisations are discussed. The first chapter contains a short review of different methods to parametrize the roughness length dependent on wave information. Based on the review it is recommended to use a parametrization based on the wave-induced stress for a planned exploratory investigation of coupled atmosphere-wave effects.

In the second chapter wave information obtained from a wave model run on the North Sea is used to derive wave-dependent fields of roughness lengths and drag coefficients. As expected, the wave-dependence introduces scatter into the relation between wind speed and drag coefficient, which is absent in the non-wave dependent parametrization. However, the scatter is small, and the correlation between drag coefficient and wind speed remains high. Therefore, no large-scale effects on the modelled drag or modelled wave and surge heights are to be expected. At least for the North Sea, coupling effects between waves and atmosphere appear not to be important.